Glycerophospholipids present in a living body of mammals, such as humans, comprise:
(1) phospholipids having an ester bond at the sn-1 position of the glycerol backbone (diacyl phospholipids); and
(2) phospholipids having an ether bond at the sn-1 position (ether phospholipids).
The ether phospholipids comprise:
(1) phospholipids having an ether bond at the sn-1 position (alkyl acyl phospholipids); and
(2) phospholipids having a vinyl ether bond (alkenyl acyl phospholipids).
Those having a vinyl ether bond (alkenyl acyl phospholipids) are also referred to as plasmalogens. A general formula of plasmalogens is described below:

In the above formula, R1 and R2 represent an aliphatic hydrocarbon group.
R1 at the sn-1 position is usually an aliphatic hydrocarbon group having 1 to 20 carbon numbers, such as dodecyl group, tetradecyl group, hexadecyl group, octadecyl group, icosanyl group, etc.
R2—CO at the sn-2 position is usually an polyunsaturated alkyl carbonyl group derived from polyunsaturated fatty acids, such as octadecadienoyl group, octadecatrienoyl group, icosatetraenoyl group, icosapentaenoyl group, docosatetraenoyl group, docosapentaenoyl group, docosahexaenoyl group, etc.
X at the sn-3 position represents a polar group in the above formula. Preferably, X is ethanolamine, choline, serine, inositol or glycerol.
In ether phospholipids of a mammal such as a human, alkyl acyl phospholipids are present in a very few amounts, while most are alkenyl acyl phospholipids, i.e., plasmalogens. Moreover, in such a mammal, there are a few plasmalogens having serine or inositol, while most of them have ethanolamine or choline. Furthermore, in a living body, the ether phospholipids are mainly contained in a biological membrane (cell membrane), so are other phospholipids.
The amount of ether phospholipids, such as plasmalogens, for example, as shown in Patent Document 1, is often measured by using red blood cell samples.
Recently, it was reported to suggest that the amount of plasmalogens in serum is low in patients with severe disease such as dementia, and Alzheimer's disease (for example, see Non-Patent Document 1). Furthermore, it is suggested that the amount of plasmalogens in serum is lower in patients with metabolic syndrome such as diabetes, arteriosclerosis and other chronic diseases.